Male Wistar rats were studied after caloric restriction by intermittant feeding alternating on a daily basis with free access to feed, (EOD), and compared to age matched controls. Morphology of left ventricular cells was determined by methods developed by Dr. Rakusan on tissue perfused in situ in our laboratory with buffered gluteraldehyde. Subsequent light micriscopic measurement showed both mean and median % tissue as capillaries are not significantly reduced by EOD feeding after 24 months on trial, as compared to adult controls on normal diet (5.46% vs. 4.98%) but there is a significant reduction in 30 month animals (3.3%) with p Less than .02 comparing 30 month EOD to 24 month, and p Less than .05 compared to adults. Cell diameter is significantly reduced with increasing age. Adult myocardial cells average 28.9 Mum, compared to 25.76 Mum for 24 month EOD, pLess than .02. A further reduction to a mean of 23.2 Mum was found at 30 months in EOD animals, (comparing 24 month EOD to 30 month EOD, p Less than .05). Isolated trabecular muscle responsiveness to superfused catecholamines appears to be preserved throughout the age range, while the senescent control animals at 24 months show a loss in responsiveness consistant with reports from this and other laboratories Thus, EOD feeding appears to protect function, preventing the reduction in responsiveness to catecholamines even though the contraction duration which is a hallmark of cardiac muscle aging is quantitatively identical in the 20 month controls and 24 month EOD muscles. Morphologically there is no difference in the two groups of animals at 24 months except for reductions in the percentage of tissue as capillaries and in the diameter of cardiac muscle cells.